In recent years, attention is being given to photocatalysts that adsorb and decompose environmental pollutants under solar light or indoor light to thereby remove the pollutants and that exhibit a self-cleaning effect on smudges adhering to their surface under solar light or indoor light, and intensive research is being conducted on such photocatalyst materials.
Titanium oxide, which is one of photocatalyst raw materials, has a high photocatalytic effect and is widely studied. However, since the bandgap of titanium oxide is large, it absorbs ultraviolet light but does not absorb visible light. Therefore, although titanium oxide exhibits photocatalytic activity under ultraviolet light, it does not exhibit photocatalytic activity under visible light, so that a photocatalyst material prepared using titanium oxide as a raw material cannot exhibit its photocatalytic effect in an indoor environment in which the amount of ultraviolet light is extremely small.
Accordingly, research and development is being conducted on the photocatalytic effects of photocatalyst materials using raw materials other than titanium oxide, and one of such materials is tungsten oxide. Tungsten oxide has a smaller band gap than titanium oxide and can absorb visible light. However, the photocatalytic activity of tungsten oxide alone is small.
It is generally considered that, when a photocatalyst material is excited, reactive oxygen species are generated by the oxidation reaction of water by holes generated in the valence band and the reduction reaction of oxygen by electrons generated in the conduction band and these reactive oxygen species oxidize and decompose organic materials. However, since the conduction band of tungsten oxide is lower than the redox level of oxygen, electrons do not contribute to the reduction reaction of oxygen but are recombined with holes, and therefore tungsten oxide alone does not exhibit high photocatalytic activity.
PTL 1 and PTL 2 disclose that tungsten oxide particles can be used as a visible light responsive photocatalyst. Specifically, this is achieved by forming an electron-withdrawing material (a promoter) on the surface of the tungsten oxide particles. In this case, the recombination of electrons excited in the conduction band under irradiation with light and holes generated in the valence band is suppressed, so that the photocatalytic activity of the tungsten oxide particles is increased.
PTL 3 discloses a photocatalyst prepared by coating the surface of tungsten oxide with titanium oxide. This photocatalyst has higher photocatalytic action than uncoated tungsten oxide and therefore can rapidly decompose volatile aromatic compounds in a vapor phase.